Techniques for dynamically switching between synchronous and asynchronous operation modes in body area networks

ABSTRACT

Techniques for dynamically switching between synchronous and asynchronous operation modes in body area networks (BANs). One embodiment includes a method ( 400 ) for transitioning from an asynchronous mode to a synchronous mode in a medium access control (MAC) of a BAN. A device receiving a switch mode command (S 410 ) performs the acts comprising: setting a timer to a time period specified in the switch mode command (S 430 ); propagating the received switch mode command to other devices in the BAN (S 440 ); upon expiration of the time period, tuning to a channel selected from a set of channels reserved for the synchronous mode (S 460 ); and initializing a device to operate as either a master device or a slave device in the synchronous mode of operation (S 470 ). Another embodiment includes a method ( 300 ) for transitioning from a synchronous mode to an asynchronous mode in a MAC of a body area networks (BAN).

This application claims the benefit of U.S. Provisional Application No.61/087,751 filed on August 11.

The invention generally relates to techniques to switch operation modesin body area networks (BANs).

A body area network (BAN) is primarily designed for permanent monitoringand logging of vital signs. An exemplary BAN, shown in FIG. 1, includesmultiple slave devices 120 which are typically sensors that can beeither wearable or implanted into the human body. The slave devices 120monitor vital body parameters and movements, and communicate with eachother over a wireless medium. The slave devices 120 can transmit datafrom a body to one or more master devices 130 from where the data can beforwarded, in real-time, to a hospital, clinic or elsewhere over a localarea network (LAN), a wide area network (WAN), a cellular network, andthe like.

One of the important factors in designing a BAN is the energy efficiencyof slave devices 120 and/or master devices 130. Efficient energyconsumption can be achieved by optimally duty cycling a receiver device(i.e., a device receiving data) between a listen state and a sleepstate. The radio of a device is turned off when the device neithertransmits nor receives data, thereby reducing the energy consumption ofthe device. A duty cycling is performed by a medium access control (MAC)protocol with the aim of minimizing idle listening, overhearing times,collisions of data transmissions and controlling overhead, whichultimately leads to power savings.

In the related art, several synchronous and asynchronous MAC dutycycling techniques are disclosed. A synchronous duty cycling includesperiodically advertising sleep and wake-up schedules, and synchronizingawake times of receiver devices. This requires an explicitsynchronization mechanism, such as beacons, to synchronize clocks andadvertise schedules. MAC protocols, for example, SMAC, TMAC and DSMAC,and the like employ explicit synchronization mechanisms to synchronizetheir active times. The SMAC protocol is disclosed in “Medium AccessControl with Coordinated Adaptive Sleeping for Wireless Sensor Networks”by Wei Ye, John Heidemann and Deborah Estrin in IEEE/ACM Transactions onNetworking, Vol. 12, No. 3, June 2004, pp 493-506. The TMAC protocol isdescribed by Tijs van Dam and Koen Langendoen in “An Adaptive EnergyEfficient MAC Protocol for Wireless Sensor Networks” in the Proceedingsof ACM SenSys, November 2003, Los Angeles. The DSMAC is discussed byPeng Lin, Chunming Qiao and Xin Wang in “Medium Access Control With ADynamic Duty Cycle For Sensor Networks” in the Proceedings of the IEEEWCNC 2004, pp. 1534-1539.

In a synchronous duty cycling MAC protocol, the active times of devicesare synchronized, whereby the broadcasting and multicasting of data issimple and efficient. This technique is also suitable for mediumreservation, mobility support and co-existence support. Accordingly,synchronous mechanisms are inherently suitable to guarantee quality ofservice (QoS) support. However, periodic synchronization incurs highoverhead, which increases the energy consumption. In addition, senderdevices (i.e., devices sending data) typically try to send their packetsduring a globally synchronized active time, such that the probability ofcollision increases. If a potential sender device loses the contention,the next opportunity arrives during the next active time. Moreover, in amulti-hop communication, after a forwarding device (a device on the pathbetween a source and destination device) receives a data packet, it mustwait until the next active time arrives before attempting to transmitthat packet. Thus, per hop the latency in a synchronous duty cyclingtechnique is relatively high.

In an asynchronous duty cycling, a sender device and a listener device(i.e., a device that listens to the medium) have independent sleep andawake times, whereby an explicit synchronization mechanism is notrequired. A preamble sampling technique is widely used in asynchronousduty cycling MAC protocols, such as WiseMAC, B-MAC and X-MAC. TheWiseMAC is further described in “WiseMAC: An Ultra Low Power MACProtocol for the Downlink of Infrastructure Wireless Sensor Networks” byEl-Hoiydi, et al. published in the Proceedings of the Ninth IEEESymposium on Computers and Communication, ISCC'04, pages 244-251,Alexandria, Egypt, June 2004. The B-MAC is described in “Versatile LowPower Media Access for Wireless Sensor Networks”, ACM SenSys November2004 by Polastre, et al., and the X-MAC protocol is published in “X-MAC:A Short Preamble MAC Protocol for Duty-Cycled Wireless Sensor Networks”ACM SenSys 2006, by Buettner, et al.

As illustrated in FIG. 2, in the preamble sampling technique all thedevices periodically listen to the medium for a short duration of time“T_(L)” and then return to a sleep state for the duration of a time“T_(CI)”, if the medium is idle. The time T_(CI)—between two successivelisten times T_(L)—is a check interval. The combination of timeintervals T_(CI) and T_(L) is a wake-up time interval. When a senderdevice has data to deliver, it transmits a wake-up (WUP) message 210that is longer than the check time interval T_(CI), of a receiverdevice. In the preamble sampling technique a WUP message 210 carriesonly preambles and does not carry any other information. When a receiverdevice wakes-up, it senses the medium and detects the WUP message 210.This forces the receiver device to stay awake until the data is fullyreceived and/or the medium becomes idle again.

The length of the WUP message 210 must be longer than the check intervalT_(CI) to ensure that a receiver device is awake when actual data istransmitted. If a check interval T_(CI) of a receiver device is verylong, then WUP message transmissions can occupy the medium for a verylong time, thereby preventing other devices from accessing the medium.

The main advantage of an asynchronous duty cycling is its simplicity, asthere is no synchronization overhead which may result in improved energyefficiency. Devices can update their wake up schedules independently. Inlow duty cycle networks, the snoop time intervals of devices aredistributed in time which reduces the probability of collision andlatency. A snoop time interval is the minimum duration during which allthe devices must listen to the medium after they wake up. However,broadcast/multicast is inefficient in the asynchronous mode and, inaddition, medium reservations cannot be supported. As a result, the QoSthat can be provided and guaranteed in such an operation mode islimited.

As can be realized from the above discussion the asynchronous andsynchronous duty cycling techniques independently achieve optimalperformance in certain scenarios. Therefore, it would be desirable toprovide a solution that could improve the performance of the BAN bydynamically switching between these different techniques as deemedsuitable by the application.

Certain embodiments of the invention include a method for transitioningfrom a synchronous mode to an asynchronous mode in a medium accesscontrol (MAC) of a body area networks (BAN). The method comprisesreceiving, by each device, a switch mode command that causes: setting atimer to a time period specified in the switch mode command; uponexpiration of the time period, tuning the device to a channel selectedfrom a set of channels reserved for the asynchronous mode; and causingthe device to operate in the asynchronous mode over the reservedchannel.

Certain embodiments of the invention also include a method fortransitioning from an asynchronous mode to a synchronous mode in amedium access control (MAC) of a body area network (BAN). The methodcomprises receiving, by each device, a switch mode command that causes:setting a timer to a time period specified in the switch mode command;propagating the received switch mode command to other devices in theBAN; upon expiration of the time period, tuning to a channel selectedfrom a set of channels reserved for the synchronous mode; andinitializing a device to operate as either a master device or a slavedevice in the synchronous mode of operation.

The subject matter that is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other features and advantages ofthe invention will be apparent from the following detailed descriptiontaken in conjunction with the accompanying drawings.

FIG. 1 is a schematic diagram of a body area wireless network.

FIG. 2 is a diagram for illustrating the operation of asynchronous dutycycling technique.

FIG. 3 is a flowchart for describing the method for transitioning from asynchronous mode to an asynchronous mode implemented in accordance withone embodiment of the invention.

FIG. 4 is a flowchart for describing the method for transitioning froman asynchronous mode to a synchronous mode implemented in accordancewith one embodiment of the invention.

It is important to note that the embodiments disclosed by the inventionare only examples of the many advantageous uses of the innovativeteachings herein. In general, statements made in the specification ofthe present application do not necessarily limit any of the variousclaimed inventions. Moreover, some statements may apply to someinventive features but not to others. In general, unless otherwiseindicated, singular elements may be in plural and vice versa with noloss of generality. In the drawings, like numerals refer to like partsthrough several views.

The asynchronous and synchronous duty cycling techniques utilized by theMAC protocol are also referred to as operation modes of the BAN. Thus,the synchronous and asynchronous MAC duty cycling techniques will bereferred to as synchronous and asynchronous modes, respectively.

Applications executed in a BAN can switch between synchronous andasynchronous modes to benefit from the advantages of each mode accordingto the requirements of the application. This is important in BANs, assuch networks are intended to support a wide range of applications whichhave diverse sets of requirements. For example, devices may switch to asynchronous mode when guaranteed QoS is a requirement of theapplication, and then switch back to an asynchronous mode for anultra-low power operation. Consider a typical BAN scenario in which awearable master device manages a few implanted slave devices. When apatient takes a shower, the master device may not be in directcommunication range with the implanted slave devices. In this scenario,an asynchronous mode can be used to provide the basic communicationcapabilities among the implanted devices. Asynchronous mode can also beused as a wake-up or a start-up mechanism. Once the devices are awakeand the network is formed, the devices may switch to the synchronousmode. As another example, devices may switch to the asynchronous modewhen a master device battery is critically low or a master devicedisappears, and no other device can play the role of the master.

The techniques for dynamically switching between modes, as disclosed inaccordance with certain embodiments of the invention, are performed by aMAC protocol that provides the hooks for scheduling and coordinating themode transition, where the decision to trigger a mode switch isperformed by a higher layer of a communication protocol (e.g., theapplication layer). As disclosed below, switching from one mode toanother often requires tuning the radio to a different channel orchannels in order to prevent devices operating in synchronous andasynchronous modes from coexisting on the same channel. With this aim,frequency bands or channels are grouped into two sets: one setrestricted to devices operating in the synchronous mode, and the otherset is restricted to devices operating in the asynchronous mode. Thesetwo sets are mutually disjoint.

FIG. 3 shows an exemplary and non-limiting flowchart 300 for describingthe method for transitioning from a synchronous mode to an asynchronousmode implemented in accordance with one embodiment of the invention. ABAN typically includes two types of devices: masters and slaves. In apreferred embodiment, a master device communicates with a set of itsslave devices using local beacons, while master devices can communicatewith each other using global beacons.

At S310 a device receives a switch mode command from a peer device orfrom a higher layer. The switch mode command may include, for example, atime when the new mode becomes effective, the channel (frequency band)that should be used for the asynchronous mode of operation, and theidentity of the group/subgroup of devices that should switch the mode.In one embodiment, the value of time can be specified as an offset oftime from the current time when the mode switch should be executed.

At S320 the device receiving the command checks whether it belongs tothe group identified in the switch mode command, and if so executioncontinues with S330; otherwise, execution ends as the device is notrequired to change its mode. That is, devices which do not belong to thegroup identified in the switch mode command continue their synchronousmode of operation on the same channel.

At S330 each device that received the switch mode command and intends toswitch to the asynchronous mode sets up a timer to the amount of timespecified in switch mode command. The purpose of delaying the modeswitching is to allow sufficient time for the switch mode command topropagate through the entire network and to prepare for the modeswitching. The value of the offset of the time specified in the switchmode command is updated as the command propagates to reflect the exacttime when the new mode becomes effective. When the specified time iselapsed, the devices receiving the command and intending to switch toasynchronous mode simultaneously switch to the asynchronous mode ofoperation, thereby minimizing the disruption of service.

Optionally, at S340 the master device broadcasts or multicasts theswitch mode command in a global beacon to coordinate the change in themode of operation with peer master devices, i.e., master devicesconnected with each other in a peer-to-peer connection. Each masterdevice receiving the switch mode command from a peer device or a higherlayer and intending to switch to the asynchronous mode may inform itsslave devices about an upcoming mode change via a switch mode commandsent in its local beacons.

At S350 it is checked if the time period measured by the timer hasexpired, and if so execution continues to S360; otherwise, executionwaits at S350. At S360 each device is tuned to a channel identified inthe switch mode command (i.e., one of the channels in the set ofasynchronous channels), and thereafter at S370 the devices startperforming their routine tasks in the asynchronous mode.

In accordance with an embodiment of the invention, these tasks includesending a wake-up (WUP) message from a sender device to one or moretarget receiver devices when a wireless medium of the BAN is free;determining if at least one target receiver device responds with a READYmessage during a sniff time interval of the sender device; determiningif a number of WUP messages transmitted by the sender device exceeds apredefined threshold, when the READY message was not received; andsetting the sender device to operate in a TURN mode when the number ofWUP messages exceeds the predefined threshold. This mode of operation isdescribed in greater detail in a co-pending application entitled “DutyCycling Techniques in Medium Access Control (MAC) Protocols for BodyArea Networks.” A sniff time interval is a fixed duration of time aftertransmitting a WUP message, during which a sender device listens to themedium.

It should be noted that during operation of the BAN one or more slavedevices may not receive the switch mode command from their master(s),and/or one or more master devices may silently disappear. This may, forexample, be due to interference/collisions on the channel or failure ofthe master device. To handle such situations slave devices that have notreceived beacons from their master device for a predefined period oftime scan the channel to detect beacons from another master device towhom they can join. If the slave devices do not find a master device towhich they can join within the predefined period of time, then slavedevices conclude that the master device has disappeared and theytransition to an asynchronous mode of operation. In this case, all theslave devices transition to a default (prearranged) channel to avoidambiguity.

FIG. 4 shows an exemplary and non-limiting flowchart 400 for describingthe method for transitioning from an asynchronous mode to a synchronousmode implemented in accordance with one embodiment of the invention.

At S410 a device receives a switch mode command from a peer device orfrom a higher layer (e.g., an application layer). The command mayinclude, for example, a future time when the new mode becomes effective,the channel identified for the synchronous mode of operation (among thechannels reserved for the synchronous mode of operation), and theidentities of the group of devices to whom the switch mode command isintended. In one embodiment of the invention, the value of time can bespecified as an offset of time from the current time when the modeswitch should be performed.

At S420, a device receiving the command checks whether it belongs to thegroup identified in the switch mode command, and if so executioncontinues at S430; otherwise, execution terminates as the device is notrequired to switch its mode of operation. That is, devices which do notbelong to the group identified in the switch mode command continue theiroperation in the asynchronous mode on the same channel.

At S430 each device intending to switch to the synchronous mode sets upa timer to the amount of time specified in the received switch modecommand. The purpose of delaying the switching mode is to allowsufficient time for the switch mode command to propagate through theentire network and to prepare for mode switching. The value of theoffset of time specified in the outgoing switch mode command is updatedto reflect the exact time when the mode switch is to be executed. Whenthe specified time has elapsed, devices intending to switch their modessimultaneously switch to the synchronous mode of operation, therebyminimizing the disruption of service.

At S440, the device receiving the switch mode command may broadcast ormulticast the switch mode command to coordinate the change in the modeof operation with its peer devices, i.e., devices connected with eachother in a peer-to-peer connection. At S450 it is checked if the timemeasured by the timer has expired, and if so execution continues withS460; otherwise, execution waits at S450.

At S460, each device ready to switch the mode is tuned to the channelidentified in the switch mode command, i.e., one of the channels in theset of synchronous mode channels. At S470, the devices are initializedto their predefined roles (e.g. master or slave) as implemented in thesynchronous mode of operation of the MAC protocol. At S480, each masterdevice scans the new channel to determine if the channel is occupied byother master devices. That is, each transitioning master device checksfor other global beacons in the channel that is now tuned to. At S490 itis checked if the channel is occupied by other master devices, and ifso, at S492 transitioning master devices join the existing beacon groupand start transmitting their global and local beacons; otherwise, thechannel is vacant, and at S494 the transitioning master device starts anew time round and transmits its global beacons. The master devices thattransition subsequently will find the channel occupied and join thefirst master at S492.

In a preferred embodiment of the invention the access to the medium isdivided into fixed and repeated duration time rounds, where a time roundis a data structure designed to include a predefined number ofsuperframes each of which includes a fixed number of time slots.

It should be noted that when a slave device switches the mode, it scansthe channel to detect local beacons of master devices to which the slavedevice can join. Once such a master device is detected, the slave devicejoins this master device and continues tracking its local beacons.

As stated above during the operation of the BAN one or more devices maynot receive the switch mode command due to message collisions orinterference on the channel, whereby these devices will not switch tothe synchronous mode. To prevent this failure, the master deviceverifies whether all of its neighbor devices have switched to thesynchronous mode. If one or more neighbor devices have not transitioned,then during its idle time the master device switches back to theasynchronous mode (for the duration of the idle time) to rebroadcast orre-multicast the switch mode command.

The principles of the invention may be implemented as hardware,firmware, software or any combination thereof. Moreover, the software ispreferably implemented as an application program tangibly embodied on aprogram storage unit or computer readable medium. The applicationprogram may be uploaded to, and executed by, a machine comprising anysuitable architecture. Preferably, the machine is implemented on acomputer platform having hardware such as one or more central processingunits (“CPUs”), a memory, and input/output interfaces. The computerplatform may also include an operating system and microinstruction code.The various processes and functions described herein may be either partof the microinstruction code or part of the application program, or anycombination thereof, which may be executed by a CPU, whether or not suchcomputer or processor is explicitly shown. In addition, various otherperipheral units may be connected to the computer platform such as anadditional data storage unit and a printing unit.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the principlesof the invention and the concepts contributed by the inventor tofurthering the art, and are to be construed as being without limitationto such specifically recited examples and conditions. Moreover, allstatements herein reciting principles, aspects, and embodiments of theinvention, as well as specific examples thereof, are intended toencompass both structural and functional equivalents thereof.Additionally, it is intended that such equivalents include bothcurrently known equivalents as well as equivalents developed in thefuture, i.e., any elements developed that perform the same function,regardless of structure.

The invention claimed is:
 1. A method for transitioning from asynchronous mode to an asynchronous mode in a medium access control(MAC) of a body area network (BAN) upon receiving a switch mode command,comprising: setting a timer to a time period specified in the switchmode command for switching from the synchronous mode to the asynchronousmode within the BAN; upon expiration of the time period, tuning a deviceto a channel selected from a first set of channels in the BAN reservedfor the asynchronous mode, wherein the first set of channels isdifferent from a second set of channels that is reserved for thesynchronous mode in the BAN; and causing the device to operate in theasynchronous mode over the reserved channel, wherein the device isconfigured for asynchronously duty cycling between sleep and wakeuptimes.
 2. The method of claim 1, further comprising receiving the switchmode command at the device and propagating the received switch modecommand to other devices in the BAN.
 3. The method of claim 1, whereinthe switch mode command includes at least a time when a mode switch isto be performed, the channel for the asynchronous mode of operation, andidentity of a group of devices that should switch their mode.
 4. Themethod of claim 2, wherein the switch mode command is sent from either apeer device or an application layer.
 5. The method of claim 4, furthercomprising checking by the device receiving the switch mode command ifthe device belongs to the group of devices designated in the switch modecommand.
 6. The method of claim 1, further comprising tuning each slavedevice operating in the asynchronous mode to a prearranged channel beingselected from the first set of asynchronous mode channels, when theslave device cannot communicate with a master device for a predefinedduration of time.
 7. A non-transitory computer readable medium havingstored thereon computer executable code, when executed, causing acomputer or a processor to perform the process of transitioning from asynchronous mode to an asynchronous mode in a medium access control(MAC) of a body area network (BAN), comprising: setting a timer to atime period specified in a switch mode command for switching from thesynchronous mode to the asynchronous mode within the BAN, upon receivingthe switch mode command at a device; upon expiration of the time period,tuning the device to a channel selected from a first set of channels inthe BAN reserved for the asynchronous mode, wherein the first set ofchannels is different from a second set of channels that is reserved forthe synchronous mode in the BAN; and causing the device to operate inthe asynchronous mode over the reserved channel, wherein the device isconfigured for asynchronously duty cycling between sleep and wakeuptimes.
 8. A method for transitioning from an asynchronous mode to asynchronous mode in a medium access control (MAC) of a body area network(BAN) upon receiving a switch mode command, comprising: setting a timerto a time period specified in the switch mode command for switching fromthe asynchronous mode to the synchronous mode within the BAN;propagating the received switch mode command to other devices in theBAN; upon expiration of the time period, tuning to a channel selectedfrom a second set of channels in the BAN reserved for the synchronousmode, wherein the second set of channels is different from a first setof channels that is reserved for the asynchronous mode in the BAN; andinitializing a device to operate as either a master device or a slavedevice in the synchronous mode of operation, wherein the device isconfigured for synchronously duty cycling between sleep and wakeuptimes.
 9. The method of claim 8, wherein the switch mode commandincludes at least a time when a mode switch is to be performed, thechannel for the synchronous mode of operation, and identity of a groupof devices that should switch their mode.
 10. The method of claim 9,wherein the switch mode command is sent from either a peer device or anapplication layer.
 11. The method of claim 10, further comprisingchecking by the device receiving the switch mode command if the devicebelongs to the group of devices designated in the switch mode command.12. The method of claim 8, wherein each device initialized as a masterdevice further performs: scanning the channel reserved for thesynchronous mode to detect at least global beacons transmitted by the atleast one other master device; joining an existing beacon group of theat least one other master device, when the channel is occupied by atleast one other master device; and transmitting global beacons on thechannel by the master device switched to the synchronous mode, when thechannel is not occupied by the at least one other master device.
 13. Themethod of claim 8, further comprising, when each device is initializedas a slave device, scanning the channel reserved for the synchronousmode to detect at least local beacons transmitted by at least one masterdevice; and joining the at least one master device when local beaconsare detected on the channel.
 14. The method of claim 13, furthercomprising checking by a master device if all its respective neighbordevices have switched to the synchronous mode; and switching back to theasynchronous mode during idle time to transmit the switch mode commandfrom the master device to its respective neighbor devices when at leastone neighbor device has not switched to the synchronous mode.
 15. Anon-transitory computer readable medium having stored thereon a computerexecutable code, when executed, causing a computer or a processor toperform the process of transitioning from an asynchronous mode to asynchronous mode in a medium access control (MAC) of a body area network(BAN), comprising: setting a timer to a time period specified in aswitch mode command for switching from the asynchronous mode to thesynchronous mode within the BAN, upon receiving the switch mode commandat a device; propagating the received switch mode command to otherdevices in the BAN; upon expiration of the time period, tuning to achannel selected from a second set of channels in the BAN reserved forthe synchronous mode, wherein the second set of channels is differentfrom a first set of channels that is reserved for the asynchronous modein the BAN; and initializing a device to operate as either a masterdevice or a slave device in the synchronous mode of operation, whereinthe device is configured for synchronously duty cycling.
 16. A wirelessdevice operable in a body area network, comprising: a radio transceiverconfigured to receive and transmit radio signals over a medium of thebody area network; a processor; a memory for storing instructions that,when executed, configure the processor to: set a timer to a time periodspecified in a switch mode command for switching from the synchronousmode to the asynchronous mode within the BAN, upon receiving the switchmode command at the device; tune a device to a channel selected from afirst set of channels in the BAN reserved for the asynchronous mode,upon expiration of the time period, wherein the first set of channels isdifferent from a second set of channels that is reserved for thesynchronous mode in the BAN; and cause the device to operate in theasynchronous mode over the reserved channel, wherein the device isconfigured for asynchronously duty cycling between sleep and wakeuptimes.
 17. A wireless device operable in a body area network,comprising: a radio transceiver configured to receive and transmit radiosignals over a medium of the body area network; a processor; a memoryfor storing instructions that, when executed, configure the processorto: set a timer to a time period specified in a switch mode command forswitching from the asynchronous mode to the synchronous mode within theBAN, upon receiving the switch mode command at the device; propagate thereceived switch mode command to other devices in the BAN; tune to achannel selected from a second set of channels in the BAN reserved forthe synchronous mode, upon expiration of the time period, wherein thesecond set of channels is different from a first set of channels that isreserved for the asynchronous mode in the BAN; and initialize a deviceto operate as either a master device or a slave device in thesynchronous mode of operation, wherein the device is configured forsynchronously duty cycling.